LXRT has 16 extensions slots. See Documentation/README.EXTENDING_LXRT on how
to use them.

Those already used by modules found in this distributions are:
0  - lxrt itself (hardwired),
1  - tasklets    (hardwired),
2  - net_rtcom   (hardwired),
3    usi         (hardwired),
8  - POSIX message queues 
9  - kcomedi_lxrt,
10 - fifos_lxrt,
11 - bits,
12 - tbx,
13 - lock_task, NI boards support
14 - rt_com_lxrt,
15 - rt_leds.

So you can reuse the above slots only if you are not using the related modules
found in this distribution. 
In any case check the value returned by: set_rt_fun_ext_index.

N.B. (hardwired) above means they are assumed as basic RTAI proper extensions.
Nonetheless extensions 1-2 do check if they can install on their hardwired 
slots, exiting their installation in error, by using back doors installed in 
LXRT. So you can use the related slots if neither tasklets nor net_rpc is being
used by your applications, __NEVER__ use 0. In any case you'd better use the 
other many free slots to avoid any trouble.
